The inadvertent introduction of ferromagnetic objects into the space surrounding high strength magnets such as magnetic resonance imaging magnets and magnets used in magnet guided catheterisation presents an ongoing hazard to both patients and staff in clinical imaging facilities. Such objects, if brought too close to the large magnet that encompasses a patient during scanning, can effectively become missiles that can seriously injure or even kill patient and staff alike, and tragically, more than one fatality has occurred. It is believed that no device exists to protect against the danger. The term “high strength” used herein is intended to include any magnet which has a field strength sufficient that ferromagnetic objects brought into the field can become dangerous.
While normal metal detectors can detect ferromagnetic objects, they also detect non-magnetic metal that can, and routinely is, safely brought up to the magnet. Such detectors therefore give a large number of false alarms and thereby lose any semblance of protective effectiveness.
Current detectors of only ferromagnetic objects assume free-standing use in a non-magnetic environment. Generalising, a first design genre aims to generate a time-varying, low audio-frequency, magnetic field and to observe the changes in that field (via the changes in the inductance of the generating system) caused by the introduction of a ferromagnetic object. However, the creation of such magnetic fields close to an imaging system is proscribed as it interferes with the imaging procedure. A second genre aims to detect changes in the earth's field when a ferromagnetic object moves close by. For such a detection method to have sufficient sensitivity, ferrites are used to enhance the voltages induced in detection coils. However, unless such a detector is to be situated an inconvenient distance from the magnetic resonance imaging suite, the fringe field of the imaging magnet will tend to saturate the ferrites, resulting in greatly impaired efficiency. Further, the presence of a large ferromagnetic object in the magnet suite, that is the detector itself, is potentially dangerous and therefore also proscribed.